This invention relates generally to liquid crystal (LCD) displays of the type commonly used in calculators and computers and, more particularly, to multiplexed liquid crystal displays in which many display elements or pixels are driven by each row and column signal line. The intersection of each type of signal line generates one display element or pixel which can be controlled independently of the other display elements that comprise a matrix of such display elements. Each pixel responds to the RMS voltage difference between the row and column signals to that pixel. A higher RMS voltage difference applied to a pixel results in turning that pixel on harder, thereby making it appear darker to the user.
Typically, six different voltages are used to drive a 32-way or higher multiplexed liquid crystal display. Therefore, each pixel has some RMS voltage across it at all times. Two important parameters affect the appearance to the user of the display. First, the absolute value of the RMS voltage applied to an on pixel or to an off pixel basically determines the lightness or darkness of the display. In addition, an on/off ratio or bias level is defined as the ratio of the on and off waveform voltages applied to each pixel. It is desirable to maximize the on/off ratio in order to make an off pixel appear as much different as possible from an on pixel to the user. At the same time, it is important to guarantee that an off pixel does not appear dark or on to the user, but that an on pixel does appear dark to the user. The voltages required to obtain this condition are set by the manufacturer of the liquid crystal display. The state of the art in LCD manufacturing is such that in order to meet the LCD threshold voltage specification under ideal bias conditions (maximum on/off ratio) requires a high peak voltage across the pixel. The maximum peak voltage that can be safely applied to an LCD driver chip without destroying it is specified by the chip manufacturer. In many liquid crystal display systems, the peak voltage that may be permitted is limited such that the LCD threshold voltage specification and ideal bias level cannot both be maintained. This limitation on peak voltage can be one imposed by the chip manufacturer or by the user's LCD driver circuitry. It is therefore the principal object of the present invention to provide LCD compensation, operating within a peak voltage limitation, that maximizes the bias level while at the same time meeting the LCD threshold voltage specification.
One type of prior art LCD compensation technique maintains the ideal bias level (maximum on/off ratio) without controlling the threshold voltage level. In this case, the display becomes dim as peak voltage limitations are imposed. Another type of prior art LCD compensation technique maintains the threshold voltage specification for the LCD without controlling the on/off ratio. In this second case, the display contrast suffers at all times, thus making making it difficult for the user to distinguish between pixels that are off and those that are on.
The LCD compensation technique of the present invention dynamically chooses between the options of threshold voltage level and bias level in order to optimize the appearance of the display at all times. It is intended to maintain the off voltage constant even when peak voltage must be reduced below its optimum level. This is because a plot of on reflectance versus RMS voltage shows flat reflectance at each voltage extreme but rapidly changing reflectance over small central RMS voltage changes. In accordance with the illustrated embodiment of the present invention some function of the peak voltage is fed into generation of the other voltage levels employed in the waveforms applied to the LCD row and column signal lines. A peak voltage V.sub.H and a step voltage V.sub.B are the starting points for derivation of all other waveform voltage levels. A reference voltage V.sub.A is the desired voltage to be applied to the LCD based upon temperature and user's contrast setting.